On Wednesday (the day after Republican Scott Brown, an opponent of cap and trade, seized a U.S. Senate seat in Massachusetts), a new scandal broke over climate science. Faced with criticism of a widely quoted piece of analysis from its 2007 climate assessment that warned that Himalayan glaciers could melt by 2035, the U.N. Intergovernmental Panel on Climate Change (IPCC) was forced to admit to relying on dubious scientific sources, apologized and retracted its earlier estimate. That estimate of the rate of Himalayan glacier loss because of warming, which appeared in the same assessment that earned the global body a share of the Nobel Peace Prize, was "poorly substantiated," the IPCC said. (See TIME's photo-essay "Himalayan Glaciers Under Threat.")

To say the least. The controversy stems from a single paragraph in Chapter 10 of the report's second section, which claimed that glaciers in the Himalayas were receding faster than in any other part of the world, and that "if the present rate continues, the likelihood of them disappearing by the year 2035 or perhaps sooner is very high if the Earth keeps warming at its current rate." Glaciologists have been doubtful of that 2035 date since the report came out. Although they are melting, there are tens of thousands of Himalayan glaciers, and it's hard to imagine them all disappearing in less than 30 years. (Watch TIME's video "Consequences As Himalayan Glaciers Melt.")

It turns out the 2035 estimate came not from a peer-reviewed scientific paper but from an interview conducted in 1999 by New Scientist magazine with the Indian glaciologist Syed Hasnain. The article, which included a "speculative" claim by Hasnain that the Himalayan glaciers could vanish by 2035, then became part of a 2005 report by the World Wildlife Fund  and that report, apparently, became the source for the IPCC claim. For his part, Hasnain says he was misquoted in the New Scientist article and claims that he had said that only a subset of the Himalayas' glacial cover might be gone in 40 years. (In my own interviews with Hasnain for a recent TIME article on Himalayan melting, he made no mention of 2035 and emphasized the need for more field research before we could be certain just how quickly the glaciers were disappearing.)

The mistake is a black eye for the IPCC and for the climate-science community as a whole. Climate scientists are still dealing with the Climategate controversy, which involved hacked e-mails from a major British climatology center that cast doubt on the solidity of evidence for global warming. (See pictures of the effects of global warming.)

It's still not clear exactly how the error made it into the IPCC's assessment, though climate scientists point out that the document was thousands of pages long and that the Himalaya claim wasn't included in the summary of the report, which was boiled down for policymakers and received the most attention from reviewers. "Honest mistakes do happen," admits Benjamin Santer, a climate modeler at Lawrence Livermore National Laboratory. "The bulk of the science is clear and compelling and rests on multiple lines of evidence," he says, not just one case.

Indeed, while Himalayan ice will almost certainly still be here in 2035, it is definitely melting  and that will have a serious impact on the billions of people in Asia who depend at least partially on Himalayan meltwater. Yao Tandong, head of China's Institute of Tibetan Plateau Research, has done on-the-ground research on the Chinese side of the Himalayas  the world's biggest collection of ice outside the two poles  and reported last year that by the end of the century, as much as 70% of the mountain range's glaciers could disappear. And far from providing evidence against climate change, nearly all alpine glaciers worldwide that have been tracked have shown significant melting over the past several decades  often documented in photographs. "It's happening globally, in Europe, North America, China and the Himalayas," says Lonnie Thompson, a glacier expert at Ohio State University. "More than 90% of the world's glaciers are retreating. Glaciers have no political agenda." (See the top 10 scientific discoveries of 2009.)

However, while climate scientists have built a nearly airtight case that climate change is happening and that manmade greenhouse-gas emissions are the primary cause, the IPCC's error demonstrates that it is still difficult to make tight predictions about the future  especially on a regional or local level.

Beyond that, the mere appearance of scientific impropriety might be enough to turn off those who are doubtful about global warming or just doubtful that the case is strong enough to warrant passing cap and trade.

Scott Brown of Massachusetts voted for a regional cap-and-trade program two years ago as a state senator. But now he's against a federal cap-and-trade bill, and he cites the "potential tampering" by climate scientists as one reason for his change of heart. There's no evidence of such tampering, but for climate skeptics, it might not matter.